1. Field of the Invention
The present invention relates to a semiconductor laser wavelength control device for use in optical communication systems or the like to keep constant the emission wavelength of the semiconductor laser.
2. Description of the Related Art
Known semiconductor laser wavelength control devices include one in which the emission wavelength of the semiconductor laser is kept constant by controlling the temperature of the semiconductor laser. A semiconductor laser wavelength control device of this type will be described below with reference to FIG. 1.
FIG. 1 is a block diagram showing the configuration of the conventional semiconductor laser wavelength control device (with divider).
The semiconductor laser wavelength control device shown in FIG. 1 is configured of a laser diode 1, a thermoelectric cooler (TEC) 2, an optical splitter 3, a first photoelectric converter 4, a light intensity reference generating unit 5, an automatic optical power control circuit 6, a wavelength filter 7, a second photoelectric converter 8, a wavelength control target generating unit 9, a divider 10a and a temperature control circuit 11.
In this configuration, first the optical output signal of the laser diode 1 is split into two optical outputs A and B by using the optical splitter 3. The optical output A, one of these split outputs, is entered into the first photoelectric converter 4, where it is converted into an electric signal VPD1 according to its light intensity.
The electric signal VPD1 here is a signal representing the optical output intensity of the laser diode 1, which is a semiconductor laser. A semiconductor laser device used for optical communication, for instance, requires restraint on the fluctuations of the average optical output intensity of the semiconductor laser, and this requirement is met by the use of the automatic optical power control circuit 6 employing auto power control (APC).
The automatic optical power control circuit 6 compares the electric signal VPD1 supplied from the first photoelectric converter 4 and an optical output intensity control target REF1 supplied from the light intensity reference generating unit 5 and, by controlling an optical output intensity control current to the laser diode 1 as to reduce the difference between them to zero, keeps the optical output intensity from the laser diode 1 constant.
On the other hand, the other optical output B of the two split outputs from the optical splitter 3 is entered into the second photoelectric converter 8 after passing the wavelength filter 7 whose light transmissivity is dependent on the optical wavelength. An electric signal VPD2 supplied from the second photoelectric converter 8 is a signal matching the light intensity of the optical output B of the laser diode 1 having passed the wavelength filter 7, and is dependent on both the optical output intensity and the optical wavelength.
In order to obtain a desired wavelength from the laser diode 1, it is necessary to keep the ratio between the electric signal VPD2 and the electric signal VPD1 supplied from the first photoelectric converter 4 constant by varying the temperature of the laser diode 1 according to the electric signal VPD2 supplied from the second photoelectric converter 8.
Logically, in order to obtain a constant wavelength, the result of division of the electric signal VPD2 by the electric signal VPD1 (VPD2/VPD1) should be kept constant all the time.
To meet this requirement, the divider 10a is provided. The electric signal VPD2 and the electric signal VPD1 are entered into the divider 10a to compute VPD2/VPD1, and the result of this division is supplied to the temperature control circuit 11 as VOUT.
The temperature control circuit 11 controls the temperature of the laser diode 1 by regulating the temperature of the thermoelectric cooler 2, which may be a Peltier element the like, as to equalize VOUT to a wavelength control target REF2 supplied from the wavelength control target generating unit 9.
The temperature of the laser diode 1 is kept constant in this way. As the wavelength of the laser diode 1, namely the semiconductor laser is heavily dependent on the temperature of the semiconductor laser, wavelength stability can be secured for the semiconductor laser by keeping the temperature constant. Further, by adjusting the wavelength control target REF2, a desired wavelength can be obtained. In other words, in the semiconductor laser wavelength control device described above, VPD2/VPD1 can be expressed in the following equation:VPD2/VPD1=REF2
By keeping this relationship, a constant wavelength can be obtained. One of such conventional devices is described in the Japanese Patent Laid-Open No. 2001-7438 specification, for instance.
The overall reaction rate is determined as a result of the reaction rate of the thermoelectric cooler and the thermal capacity of the object of control. In a laser module for optical transmission or the like, these reaction rates can be approximated to a low pass filter (LPF), whose cut-off frequency fc is at a very low level, ranging between 0.01 Hz and 0.1 Hz approximately.
Where a thermoelectric cooler and a control circuit are to be configured of feedback circuits, the feedback loop requires a sufficient low frequency (DC) gain in order to make the temperature of the object of control identical with the target temperature. The error quantity of a system is usually proportional to the reciprocal of the gain of the loop. Where the ambient temperature is constant and the target temperature is constant, the error quantity of the system is determined by the gain in DC.
For this reason, the thermoelectric cooler and the feedback circuit to control it require consideration for securing stability and a sufficient DC gain of the loop of the whole system. Furthermore, since there are significant differences among individual thermoelectric coolers and changing the type of cooler would expand the range of the cut-off frequency fc, fluctuations in cut-off frequency fc should also be taken into account.
Conventional control systems using an object of control having this low frequency characteristic include the analog control system and the PID control system.
The analog control system, as shown in FIG. 2, is configured of a high gain stage 91 for raising the DC gain and an LPF 92.
The transmission characteristic H(s) shown in FIG. 2 in the working of this system is represented by the following Expression (1):H(s)=[A/RC]·{1/(s+1/RC)}  (1)where A is the gain.
In order to stabilize the whole loop, a phase margin should be secured. If the pole of the control circuit is sufficiently higher in frequency than the cut-off frequency fc and sufficiently higher than the frequency fc at which the open loop gain of the whole loop becomes 1, a phase margin can be secured.
However, this means that a large control signal of a high frequency is applied to the object of control and, though the output signal from the object of control is smoothed by the LPF, wild variations will occur in the object of control, which are undesirable. Therefore, the pole is set lower than or close to the cut-off frequency fc. As this results in a second loop together with the cut-off frequency fc of an electrothermal conversion element, the phase margin will pose a problem. As shown in FIG. 3, there is used a system to which a phase compensation circuit is added.
The transmission characteristic H(s) of the circuit shown in FIG. 3 is represented by the following Expression (2):
                              (                      Formula            ⁢                                                  ⁢            1                    )                ⁢                                  ⁢                              H            ⁡                          (              s              )                                =                                                    AR                2                                                              R                  1                                +                                  R                  2                                                      ·                                          s                +                                  1                                                            R                      2                                        ⁢                    C                                                                              s                +                                  1                                                            (                                                                        R                          1                                                +                                                  R                          2                                                                    )                                        ⁢                    C                                                                                                          (        2        )            
According to the PID control system, as shown in FIG. 4, the control characteristic is determined with a coefficient having three terms including a proportional term P, an integral term I and a differential term D as parameters. PID control has a high degree of freedom and is versatile. To determine the PID parameters, approximate values, namely initial setting, are determined by using an adjustment law on the basis of rough characteristics of the object of control. However, this initial setting does not always provide a fully satisfactory control characteristic, but it is often necessary to read just the parameters while controlling the object of control in this manner of control and watching the actual control characteristic. Regarding the stability of the loop, too, the parameters should be adjusted to settle the control quantity and the operation quantity to constant levels while performing actual control on the step response and the like (see for instance The Institute of Systems, Control and Information Engineers, PID Control, pp. 16-38, Asakura Shoten, September, 2002 (in Japanese)).
However, where the frequency characteristic of the object of control has a low LPF characteristic, the cut-off frequency fc is as low as fc<0.1 Hz. As the conventional analog control system shown in FIG. 3 requires a sufficiently lower phase compensation than the cut-off frequency fc, the RC constant takes on a high value, entailing an unsolved problem of difficulty of size reduction and large scale integration.
There is another unsolved problem of a long time taken for stabilization on account of a very low time constant. Furthermore, the cut-off frequency fc varies from element to element, which is the object of control, and the analog circuit itself involves no little fluctuation, making it necessary to adjust individual elements for stabilization of the whole loop, which poses an unsolved problem in productivity.
Moreover, as the control characteristic obtained for the conventional PID control system described above is evaluated with actual hardware, and the parameters should be adjusted accordingly, the differences from element to element make it difficult to determine the parameters, and accordingly the development of a design takes a long time, which is another unsolved problem. Further, when any element is to be changed to one of another type, the parameters have to be redesigned, entailing still another unsolved problem of requirement for many additional man-hours spent on development.
Next will be described the configuration of a semiconductor laser wavelength control device which does not use the divider 10a shown in FIG. 5. It may be noted here that parts shown in FIG. 5 having counterparts in FIG. 1 will be denoted by respectively the same signs, and their description will be dispensed with.
The semiconductor laser wavelength control device shown in FIG. 5 differs from the device shown in FIG. 1 in the use of a subtractor 10 in place of the divider 10a. Further, the wavelength control target generating unit 9 in this FIG. 5 is supposed to supply the target value of VPD2-VPD1 as the wavelength control target value REF2.
Thus, the subtractor 10 subtracts the electric signal VPD1 from the electric signal VPD2, and the result is supplied to the temperature control circuit 11 as VOUT. The temperature control circuit 11 controls the temperature of the laser diode 1 by regulating the temperature of the thermoelectric cooler 2 as to equalize VOUT to the wavelength control target value REF2 generated by the wavelength control target value generating unit 9. Conventional devices of this kind include, for instance, what is disclosed in the Japanese Patent Laid-Open No. 2002-270954 specification.
However, whereas the conventional semiconductor laser wavelength control device shown in FIG. 1 uses the divider 10a, the large circuit scale of this divider 10a invites a problem of expanding the overall size of the device.
Further, as both VPD2 and VPD1 which are divided by the divider 11a vary at a fixed increase rate, division of the two signals makes it possible to keep the wavelength constant as indicated by a curve 21 shown in FIG. 6. FIG. 6 shows the relationship of dependence between VPD2/VPD1 and the electric signal VPD1 supplied from the first photoelectric converter 4.
However, where VPD2 cannot be divided by VPD1 without a remainder, this remainder will give rise to a setting error, entailing a problem that the emission wavelength of the laser diode 1 cannot be controlled at a constant value.
The semiconductor laser wavelength control device shown in FIG. 5 on the other hand, as it uses the subtractor 10 in place of the divider 10a, can solve the aforementioned problems of the expanded circuit scale and the setting error due to the indivisible remainder.
In the configuration using the subtractor 10, as the wavelength control target REF2 is equal to VPD2-VPD1, VPD2/VPD1 will be as represented by the next Expression (3):
                                                                        VPD                ⁢                                                                  ⁢                                  2                  /                  VPD                                ⁢                                                                  ⁢                1                            =                                                                    (                                                                  VPD                        ⁢                                                                                                  ⁢                        1                                            +                                              REF                        ⁢                                                                                                  ⁢                        2                                                              )                                    /                  VPD                                ⁢                                                                  ⁢                1                                                                                        =                              1                +                                  (                                      REF                    ⁢                                                                                  ⁢                                          2                      /                      VPD                                        ⁢                                                                                  ⁢                    1                                    )                                                                                        (        3        )            
As indicated by this equation (3), in order to keep the wavelength constant, the electric signal VPD1 supplied from the first photoelectric converter 4 should be kept constant.
The reason is that, where VPD2-VPD1 is calculated by using the subtractor 10, when both signals vary at a fixed increase rate, there arises a difference between the varied level of VPD2 and that of VPD1, and this difference is calculated. For instance, if VPD2=2 and VPD1=1 have trebled each, they will be 6 and 3, respectively, and the calculation of 6−3 will give a different result from 2−1.
In other words, when the electric signal VPD1 has varied, there arises an error in the calculation of the electric signals VPD2 and VPD1, and it becomes impossible to keep the wavelength constant as indicated by a curve 22 in FIG. 6. For this reason, the wavelength is kept constant by having the automatic optical power control circuit 6 restrain the variations of the electric signal VPD1.
However, there is a problem that, when the deterioration of the laser diode 1 over time causes the optical output intensity control current required by the laser diode 1 to surpass the controllable range of the automatic optical power control circuit 6, the wavelength cannot be kept constant.
On the other hand, depending on the purpose for which the semiconductor laser wavelength control device is to be used, the optical output intensity of the laser diode 1 should be continuously varied. However, when the optical output intensity is continuously varied, there arises an error in the calculation by the subtractor 10 as stated above, resulting in a problem of impossibility to keep the wavelength constant.
In view of these problems, an object of the present invention is to provide a semiconductor laser wavelength control device which can control the optical wavelength to remain constant even if the optical output intensity of the semiconductor laser varies and can be reduced in overall size.
Another object of the invention is to provide a temperature control device which, where the object of wavelength control has a frequency characteristic of a low LPF characteristic, can be sufficiently stable and easy to design and permits large scale integration even if the possibility of individual differences and type change of the object of control are taken into consideration.